Glide-slope aerials or angle-of-approach aerials are generally used in combination with localizer aerials or directional aerials in an automatic landing system in aircraft. Transmitters at the beginning and the end of the landing path transmit signal beams which are received by the aerials. If the aircraft deviates from a predetermined approach course, this will be indicated by a difference in intensity in the signals received.
Glide-slope aerials are known per se from the prior art, for example from the U.S. Pat. Nos. 3,906,507 and 3,220,006. If used in an aircraft, such glide-slope aerials are generally sited in the radome space in the nose of the aircraft, which radome space is bounded at one side by a vertical surface which is perpendicular to the longitudinal axis of the aircraft. It is possible for this surface to be the front pressure bulkhead of the passenger cabin.
Glide-slope aerials are horizontally polarized, semicircular, arc-type or half-loop aerials. In other words, glide-slope aerials may be regarded as magnetic dipoles whose dipole axis is vertically directed and extends in a vertical plane through the base points of the aerial.
In principle, such a semicircular arc-type aerial sited, on an infinitely large ground plane has an omni-directional pattern in the forward direction and no depolarization component. Because the bulkhead which has to serve as ground plane for use in an aircraft is not infinitely large, but has, for example, a diameter of only 1.5 .congruent. (=1.5 wavelengths) the currents which are induced in the ground plane as a consequence of the electromagnetic radiation generated by the aerial will be deflected at the edge of the ground plane, which produces a relatively strong component in the direction of the said edge. If the aerial is in the centre of the ground plane (which is assumed for the sake of convenience to simply be symmetrical), the resulting depolarization component formed by the resulting vertical component of the edge currents would be balanced in the principal planes of symmetry, as a result of which the radiation pattern will not have any depolarization component in those directions. For other directions, however, a small component of, for example, -20 dB will be left over.
However, if one sites the aerial not in the centre of the ground plane, but if the aerial is for example pushed upwards, the current is no longer symmetrically deflected at the edge, and as a result of this the vertical components of current are no longer balanced and a depolarization component is produced in the radiation pattern even in directions situated in the principal plane of symmetry. The nearer the aerial comes to the edge of the ground plane, the stronger the effect becomes. Especially when the distance from the edge of the ground plane becomes less than 1/6 .lambda., the reactive currents around the base of the aerial are also affected. Since there is generally very little space in the radome space of an aircraft and an increasing number of aerials (for radar purposes, automatic landing systems etc.) have to be installed in the radome space, it will generally not be possible to mount the glide-slope aerial in the centre of the ground plane. In practice, however, distances from the edge of less than 1/6 .lambda. do not occur.
If two or more aerials are sited within each other's sphere of influence on the ground plane, coupling currents will start to run across the ground plane. In general, this situation will occur in practice. The coupling currents which occur affect the radiation pattern of both aerials and have, in addition, vertical components in the case where the aerials are not at the same height. Any inclination of the ground plane, the presence of stiffeners situated on the outside of the ground plane and high edges of the radome space also give rise to deformation of the radiation pattern and additional depolarization.
The effect of the stiffening components can be eliminated by fitting a flat plate in the radome space in front of the irregular structure of the stiffeners, on which plate the glide-slope aerials can be mounted. The abovementioned effects due to the (high) edge boundary of the radome space and any inclination of the pressure bulkhead are not, ,however, eliminated thereby.